1. Field of the Invention
The present invention relates to field emitters and methods of fabricating the same. More particularly, the present invention relates to forming field emission tips by the use of facet etching.
2. State of the Art
Various types of field emitters are used in a variety of devices, from electron microscopes to ion guns. However, one of the most prevalent commercial applications of field emitters is flat panel displays, such as cold cathode field emission displays (“FEDs”) used for portable computers and other lightweight, portable information display devices.
As illustrated in FIG. 18, an exemplary flat panel cold cathode FED 200 comprises a flat vacuum cell 202 having a cathode 204 and an anode 206 spaced apart from one another in a mutually parallel relationship. The cathode 204 comprises a conductive or semiconductive first material 208, such as silicon, disposed on a substrate 212, such as a semiconductive or dielectric material, and an array of minute field emission tips 214 distributed across the material 208. The anode 206 comprises a conductive second material 216 disposed on an interior surface of a transparent plate 218, and a phosphorescent or fluorescent material 222 coated on the conductive second material 216. A conductive structural element, called a gate 224, is disposed in the space between the cathode 204 and anode 206. The gate 224 is generally formed atop a grid of dielectric material 226 deposited on the cathode 204. The field emission tips 214 reside within openings in the gate 224 and in the dielectric material 226, such that the gate 224 surrounds each field emission tip 214. The gate 224 acts as a low-potential anode (i.e., lower potential than the anode 206), such that when a voltage differential, generated by a voltage source 228, is applied between the cathode 204 (strong negative charge), the gate 224 (weak positive charge), and the anode 206 (strong positive charge), a Fowler-Nordheim electron emission is initiated, resulting in a stream of electrons 232 being emitted from the field emission tips 214 toward the phosphorescent or fluorescent material 222. The electron stream 232 strikes and stimulates the phosphorescent or fluorescent material 222. The stimulated phosphorescent or fluorescent material 222 emits photons (light) (not shown) through the conductive second material 216 and the transparent plate 218 to form a visual image.
FIGS. 19–23 illustrate a conventional method of forming a field emission tip. As shown in FIG. 19, a substrate of conductive or semiconductive material 252, such as silicon, is deposited or formed over a dielectric support 254. A mask material is patterned (such as by lithography) to define a mask element 256 at the position of the emission tip 258 to be formed. The conductive or semiconductive material 252 is then etched, such as by a wet etch or an isotropic dry etch, which “undercuts” the mask element 256 to form a sharp field emission tip 258 beneath the mask element 256, as shown in FIG. 20. The mask element 256 is then removed, as shown in FIG. 21. Although such a method is commonly used to form field emission tips 258, the method has drawbacks. For example, as shown FIG. 22, if the etching is halted too soon, inefficient, blunt field emission tips 262 are formed. Further, if the etching is not halted soon enough, the mask element 256 is undermined and the field emission tips 264 formed are short and may be ineffective, as shown in FIG. 23 (shown with the mask element 256 collapsed onto the conductive or semiconductive material 252). In other words, the short field emission tip 264 may not be close enough to a gate in a field emission display to generate a sufficient stream of electrons striking the phosphorescent or fluorescent material on the anode to stimulate the material and form a visual image.
Other field emission tip formation techniques which do not involve isotropic etching are also known. For example, U.S. Pat. No. 5,312,514, issued May 17, 1994 to Kumar (“the Kumar patent”), relates to forming field emission tips by distributing a discontinuous etch mask material across an electrically conductive material layer. The discontinuity of the etch mask material forms random openings therein. The etch mask material is selected such that the electrically conductive material layer will etch at a faster rate than the etch mask material (at least twice the rate) when the electrically conductive material layer is ion etched. The ion etch is performed until all of the etch mask is removed, which results in v-shaped valleys in the electrically conductive material defining peaked field emission tips therebetween. Further, the Kumar patent discusses using a low work function material for the electrically conductive material layer and also discusses depositing a low work function material over the electrically conductive material after the formation of the field emission tips. Although the method taught in the Kumar patent eliminates the use of an isotropic etch to form field emission tips, it lacks control over the field emission tip distribution and dimensions. The discontinuous layer of etch mask material results in a nonuniform distribution of field emission tips, since the positions of the openings in the discontinuous layer cannot be controlled. Furthermore, the discontinuous layer of etch mask material results in non-uniform dimensions between the field emission tips, since the thickness difference across the discontinuous layer cannot be controlled. In other words, the field emission tips formed in areas where less etch mask material existed over the conductive material will be shorter than in other areas. Moreover, since the etch mask material is a discontinuous layer rather than a patterned mask, the size or diameter of the field emission tips formed cannot be controlled.
Thus, it can be appreciated that it would be advantageous to develop a technique which would result in novel field emission tips having uniform distribution and uniform, precise dimensions.